Referring to FIG. 1, there is shown a longitudinal sectional view of a prior art color deflection lens (DFL) CRT 50. A single beam, monochrome DFL CRT is described and claimed in application, Ser. No. 07/874,043, filed Apr. 27, 1992, now U.S. Pat. No. 5,327,044, issued Jul. 5, 1994, and entitled "Electron Beam Deflection Lens for CRT," while a multi-beam, color DFL CRT is described and claimed in U.S. Pat. No. 5,204,585, issued Apr. 20, 1993, and entitled "Electron Beam Deflection Lens for Color CRT." The present invention is applicable to the inventions described and claimed in the aforementioned patent application and issued patent, the disclosures of which are hereby incorporated by reference in the present application.
CRT 50 is of the multi-beam, or color, type and includes a sealed glass envelope 68 having a generally cylindrical neck portion 68a, a frusto-conical funnel portion 68b, and a display screen 54. Disposed in a sealed manner on an aft portion of the glass envelope's neck portion 68a is a plug-like connector 58 comprised of a plastic housing 64 and a plurality of conductive pins 72 extending in a sealed manner through a distal end of the glass envelope's neck portion. Disposed on an inner surface of display screen 54 is a phosphor layer 56 responsive to an electron beam incident thereon for providing a video image. The phosphor layer 56 is in the form of a large number of discrete phosphor elements arranged in groups of three for each of the primary colors, i.e., red, green and blue. A charged metal shadow mask 82 having a large number of apertures therein is disposed immediately adjacent to the phosphor layer 56. Each of the apertures in shadow mask 82 is aligned with a respective one of the aforementioned phosphor elements in phosphor layer 56 for allowing an electron beam to be incident upon the phosphor element as the electron beams are swept across the inner surface of display screen 54 in a raster-like manner. The charged shadow mask 82 serves as a color selection grid, ensuring that each of the three electron beams 52a, 52b and 52c (shown in dotted-line form) lands only on its assigned phosphor elements, or deposits.
Disposed within DFL CRT 50 is a multi-grid electron gun 51 including, in proceeding toward display screen 54, a low voltage beam forming region (BFR) 74, a prefocus lens 76 and a high voltage deflection focus lens 78. FIG. 2 is a longitudinal sectional view of the various charged grids of electron gun 51. Energetic electrons are emitted by three heated cathodes K.sub.R, K.sub.G and K.sub.B for each of the primary colors of red, green and blue. BFR 74 is aligned with the three cathodes to receive the energetic electrons and form these electrons into the aforementioned three electron beams 52a, 52b and 52c. BFR 74 includes a G.sub.1 control grid, a G.sub.2 screen grid and a facing portion of a G.sub.3 grid. The three electron beams 52a, 52b and 52c are then directed to the prefocus lens 76 which includes a G.sub.5 grid, a G.sub.4 grid and a facing portion of the G.sub.3 grid. The electron beams then are directed through the deflection focus lens 78 which includes a G.sub.6 grid and a facing portion of the G.sub.5 grid. Disposed about and engaging the G.sub.5 grid is a support, or convergence, cup 60. Attached to support cup 60 about its periphery are a plurality of contact clips, or bulb spacers, where two such contact clips are shown as elements 62a and 62b in FIG. 1. Contact clips 62a and 62b engage an adjacent inner surface of the neck portion 68a of the CRT's glass envelope 68 upon which is disposed a resistive coating 84. The combination of support cup 60 and contact clips 62a and 62b as well as a plurality of glass beads attached to each of the grids (which are not shown in the figure) provide secure support for electron gun 51 in CRT 50.
Within the deflection focus lens 78, the G.sub.6 grid may be in the form of either a conductive layer disposed on the inner surface of the glass envelope's frusto-conical funnel portion 68b, or may be in the form of a frusto-conical metallic element disposed immediately adjacent to the inner surface of the frusto-conical funnel portion 68b of the CRT's glass envelope 68. The G.sub.6 grid is maintained at a high anode, or accelerating, voltage, while the remaining grids in electron gun 51 are maintained at various lesser voltages for focusing the three electron beams 52a, 52b and 52c on the CRT's face-plate 54. The three electron beams 52a, 52b and 52c also pass through a beam deflection region 80 defined by a magnetic deflection yoke 66 disposed about the CRT's glass envelope 68 generally where its neck portion 68a meets its frusto-conical funnel portion 68b. Deflection yoke 66 displaces the three electron beams 52a, 52b and 52c across display screen 54 in a raster-like manner, executing a beam retrace following a complete scan of the display screen. By positioning one or more grids of the CRT's main focus lens on, or in closely spaced relation to, an inner surface of the CRT's glass envelope 68, the main focus lens may be positioned within the deflection yoke's magnetic field so as to locate the deflection center of the beams within the focal point of the main focus lens in forming a beam deflection lens. The deflection lens not only focuses the beams on the CRT's display screen 54, but also increases beam deflection sensitivity as the beam is deflected by the magnetic deflection yoke 66. Co-locating the CRT's main focus lens and beam deflection region 80 also reduces lens spherical aberration of the beams and allows for shorter CRT length as described in the aforementioned co-pending application and issued patent.
As the electron beams are deflected across the CRT's display screen 54, they are displaced from the CRT's longitudinal axis A-A'. Deflection of the electron beams from the CRT's axis gives rise to an imbalance in the symmetrical electrostatic forces applied to the beams by the various charged grids of the CRT's electron gun 51. This effect is shown in the simplified schematic diagram of FIG. 3 of a CRT 90 having a glass envelope 92 with a neck portion 92a, a funnel portion 92b and a display screen 92c. Electron beam 96 is generated and directed onto display screen 92c by an electron gun as described above which is not shown in the figure for simplicity. Electron beam 96 is disposed along the CRT's longitudinal axis B-B' in the neck portion 92a of the CRT's glass envelope 92. The deflection focus lens in CRT 90 is shown in the figure in dotted-line form as element 91 and is located in the CRT where the electron beam 96 is magnetically deflected. As electron beam 96 is deflected across faceplate 92c by a magnetic deflection yoke 94, an unsymmetrical force is applied to the electron beam in the direction of, or toward, the CRT's longitudinal axis B-B'. For example, where the electron beam is deflected upward above axis B-B' as shown for the case of electron beam 90a, a downward force F is exerted on the electron beam as shown in the figure. Similarly, where the electron beam is deflected downward below axis B-B' as shown for the case of electron beam 96b in dotted-line form, an upwardly directed force F' is exerted on the electron beam urging it toward the CRT's axis B-B'. The force exerted on the electron beam is unsymmetrical and increases with the deflection of the beam from axis B-B'. Thus, when the beam is fully deflected adjacent to an edge of display screen 92c, the axis-directed force exerted on the beam is maximum. This unsymmetrical, off-axis force gives rise to defocusing of the electron beam and an unsymmetrical electron beam spot on the CRT's display screen 92c. For example, in the case of the upwardly deflected electron beam 96a, downwardly directed force F gives rise to a teardrop-shaped electron beam spot 98a having a tail directed toward axis B-B'. Similarly, for the downwardly directed electron beam 96b, upwardly directed force F' gives rise to a teardrop-shaped electron beam spot 98b on the CRT's faceplate 92c with a tail directed toward axis B-B'. Although this discussion of beam defocusing and beam spot distortion is in terms of beam vertical deflection, a similar defocusing effect occurs when the electron beam 96b is horizontally deflected to the right and left of the CRT's axis B-B'.
FIG. 4 is a simplified plan view of the CRT's display screen 92c illustrating the manner in which defocusing of the electron beam causes electron beam spot distortion with off-axis deflection of the electron beam. For example, electron beam spots 102 and 104 which lie on the horizontal centerline of display screen 92c are teardrop-shaped with a tail directed inwardly toward the center of the display screen. Similarly, electron beam spot 100 which lies on the vertical centerline of the CRT's faceplate 92c is teardrop-shaped with a tail directed downward toward the center of the display screen. Electron beam spots 106 and 108, which are off-axis, similarly are teardrop-shaped having tails directed toward the display screen's center. Only electron beam spot 110 has the desired circular shape because it is located at the center of the CRT's display screen 92c and is undeflected from the CRT's axis.
The present invention addresses the aforementioned limitations of the prior art by providing dynamic off-axis defocusing correction for a deflection lens CRT. The present invention incorporates an unsymmetrical correction focus lens in the CRT's electron gun to correct for off-axis defocusing and provide a well defined, circular electron beam spot over the entire surface of the CRT's faceplate.